1. Field of the Invention
This invention is broadly concerned with an electric motor drive system in a so-called hybrid vehicle and particularly to a structure for regulating the rotational direction of an output shaft of the hybrid vehicle.
2. Description of the Related Art
Heretofore, various hybrid vehicles have been proposed, which are driven for running by combined or hybrid drive including an engine and an electric motor. As a typical brake system of many such hybrid vehicles, a mechanical brake, such as a disk brake or a drum brake, is provided on a drive wheel or like wheel.
For smoothly up-slope starting such a hybrid vehicle, that is, for smoothly starting the vehicle from the parking state thereof along an upward slope, it is an essential requirement to accelerate the vehicle accurately by accompanying the de-coupling of the mechanical brake. When up-slope starting the hybrid vehicle by driving the electric motor, the motor is started from the zero rpm state. This means that the vehicle is subjected to retreat or abrupt start unless the accelerator pedal is depressed in proper extent and at proper timing with respect to the timing of the brake de-coupling.
Also, when up-slope starting the hybrid vehicle by driving the engine, like the case of the up-slope start by driving the electric motor, retreat or abrupt start is liable despite steady engine rotation.
To avoid the above undesired possibilities, it has been proposed to provide the hybrid vehicle with an electric brake in addition to the mechanical brake (as disclosed in Japanese Patent Laid-Open No. 62-95903).
Such a hybrid vehicle employs an electric motor, which is power supply controlled according to a command value calculated from the accelerator pedal depression extent, brake pedal depression extent, etc. detected by an acceleration sensor, a braking sensor, etc.
For example, when the brake pedal is depressed to stop a hybrid vehicle, not only the conventional mechanical brake is rendered operative, but also a reverse current is supplied to the electric motor in accordance with the brake pedal depression extent so that the vehicle tends to run backward, i.e., the drive wheel tends to be rotated reversely. A stopping torque is thus generated on the motor output shaft connected to the drive wheel. This has an effect of braking the vehicle.
When stopping a hybrid vehicle running forward along an up-slope, a reverse current tending to block the retreat of the vehicle is applied to the electric motor. This current has to be high compared to the current supplied to the motor to stop the vehicle on a level place. This is so because the vehicle tends to be retreated due to the inclination of the slope.
When up-slope starting the hybrid vehicle, a normal or forward current is supplied to the electric motor according to a command value calculated from the accelerator and brake pedal depression extent, etc.
The hybrid vehicle thus can be started smoothly along the up-slope without possibility of retreat or abrupt start.
Meanwhile, the electric brake noted above is such that current is supplied to only in-phase coils of the electric motor in the parked state of the hybrid vehicle. This means that the heat capacity of the power supply element such as a transistor possibly becomes insufficient.
Particularly, when stopping the hybrid vehicle on a slope, a high current is supplied to the in-phase coils, thus resulting in pronounced heat capacity shortage of the transistor or like power supply element.
An object of the invention is to provide a structure for regulating the rotational direction of the output shaft of a hybrid vehicle, which permits smooth start of a hybrid vehicle along an up-slope and also permits stopping of the vehicle without possibility of the heat capacity shortage of the power supply element.